Display screen or portion thereof with a graphical user interface



The FIGURE is a front view of a display screen or portion thereof with a graphical user interface showing the new design.

The broken lines showing a display screen or portion thereof, all text, symbols, and portions of the graphical user interface shown in broken lines illustrate portions of the article that form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 